Highly pure polysilicon material is a fundamental raw material for the semiconductor and photovoltaic industries. As the development of distributed photovoltaic power generation, the photovoltaic market is growing, which will in turn promote the rapid development of the polysilicon industry. The processes for preparing polysilicon comprise modified Siemens method, metallurgy method, fluidized bed method and the like. Among others, the modified Siemens method produces more than 80% of polysilicon of the total output of the world, which method comprises the crucial production process of feeding trichlorosilane, after purification by rectification, together with highly pure hydrogen into a reactor, subjecting to a chemical vapor deposition reaction on the surface of a silicon core (the silicon core is heated to 1000-1150° C.) in the reactor, and growing gradually the silicon core to be rod-like polysilicon. The unreacted trichlorosilane, dichlorosilane, silicon tetrachloride, hydrogen and hydrogen chloride contained in the tail gas are recovered through purification by a tail gas recovery process. For the modified Siemens method, it is necessary to shutdown the reactor to take out “silicon rod” (the rod-like polysilicon product) after growing to certain size, such that the batch operation comprising starting and shutting down the reactor not only wastes abundant heat, but also decreases largely the output of the reactor.
Accordingly, the fluidized bed process is being more and more interested by those skilled persons as it is a process having a large deposition surface area, a low energy consumption for chemical vapor deposition and a continuous operation to produce polysilicon. The fluidized bed process is a process to produce polysilicon developed early by Union Carbide. The process produces a granular polysilicon product by using silicon tetrachloride (SiCl4), H2, HCl and industrial silicon as raw materials, generating trichlorosilane (SiHCl3) in a high pressure, high temperature fluidized bed (boiling bed), subjecting the SiHCl3 to a further disporportionation hydrogenation reaction to generate dichlorosilane (SiH2Cl2), followed by a disporportionation to generate silane, and feeding the silane or chlorosilane into a fluidized bed reactor added with granular silicon seed (also called as “silicon seed”) at a reaction temperature of 500-1200° C. for a continuous thermolysis reaction. According to the categories of the silicon-containing gas fed into the fluidized bed reactor, the beds are generally classified into silane fluidized bed and chlorosilane fluidized bed (e.g. trichlorosilane fluidized bed). As the granular silicon particles taking part in the reaction in the fluidized bed reactor have a large surface area, the process leads to a high production efficiency, a low power consumption and a low cost. Another advantage of the fluidized bed process is: during the downstream growth of crystals, the silicon particles can be loaded directly into a crucible where the crystals grow, while the rod-like polysilicon product produced by the conventional modified Siemens method needs to be fractured and sorted before being loaded into the crucible, and needs additionally a series of technical processes, such as etch with highly pure inorganic acid, rinse with ultrapure water, drying and treatment in clean conditions. Therefore, the rod-like polysilicon product leads to a high cost of post treatment compared with granular silicon, and contamination is readily to be involved during the post treatment.
Currently, most of the fluidized bed reactors use the external heating, i.e., a way of providing heat by externally heating the particles of the fluidized bed, such as heating by liner and/or reactor isolating layer. U.S. Pat. No. 4,786,477 discloses a microwave heated fluidized bed reactor, which microwave heats the silicon particles with a microwave generator outside the reactor. Such a method allows the reactor internal wall to have a temperature lower than that of the silicon particles, but it needs a specific microwave generator, which is costly. U.S. Pat. No. 7,029,632 discloses a radiation heated fluidized bed reactor, which radiation heats the reaction zone with a heat source peripheral to the reactor inner tube. U.S. Pat. No. 4,883,687 discloses another way of external heating. Such a way of external heating by heat radiation or heat conduction results in the temperature of the reactor greater than the temperature of the materials inside the reactor, which is readily to cause the deposition of polysilicon on the internal wall of the reactor, and to prevent the heat transferring toward inside of the fluidized bed. Accordingly, such a way of heating generally brings large energy loss to the system. In particular, such contradiction is being more significant as the large-scaling of fluidized beds, where the temperature of the reactor wall is too high and deposition of silicon occurs, whilst the temperature of the reactor center cannot reach the decomposition temperature of the silane or chlorosilane, which not only affects adversely the production efficiency of the reactor, but also makes it necessary to shutdown the reactor due to the silicon deposition on the wall for detection, cleaning or replacement, such that the production capacity is limited.
Generally, the deposition on the reactor wall can be depressed by separate the heating zone and the reaction zone. For example, US 2002/0081250 discloses a fluidized bed reactor having separately a heating zone and a reaction zone, with the heating zone positioned below the reaction zone. CN 200810116150.3 further isolates apart the heating zone and the reaction zone, so as to form an external circulation outside the reactor. The way of external heating has a significant disadvantage of poor uniformity of heating, particularly for a large size fluidized bed reactor, resulting in low heating efficiency and large temperature difference between the reactor wall and the reactor center, which leads to poor safety, tendency of deposition of silicon powder on the reactor wall and poor product purity. The way of internal heating the fluidized bed reactor also has the defect of readily depositing silicon on the heating device.
CN 201010116785.0 discloses a fluidized bed reactor with internal heating, which separate the reactor into a heating zone and a reaction zone with a guide cylinder in the reactor, wherein the heating zone heats the silicon particles through a electric-resistance heating element, and the heating zone is not fed with a silicon-containing gas, so as to decrease the wall surface deposition. CN102745692A discloses a fluidized bed reactor with internal heating, which provides heat through supplying power to a electric-resistance element extending into the fluidized bed and connecting with a pole, wherein the resistance element is made of at least one of ceramics (such as graphite, silicon carbide and the like) and metals, and the resistance heating element is equipped with an outside protective case, where the surface of the protective case has preferably a coating or lining of silicon, silica, quartz, silicon nitride or the like. CN 200780015545.8 discloses a fluidized bed reactor with internal heating, which heats through a resistance heating element at the bottom, ejects the silicon-containing gas into a reaction zone through a nozzle extending into the reaction zone, so as to avoid the deposition of silicon in the heating zone. However, the intensive back mixing performance of the fluidized bed results in a part of the silicon-containing gas into the heating zone. As the temperature of the heating element is higher than that of the gas phase and the solid particles, deposition occurs on the heating element, which will finally result in decreased heating efficiency, and even may cause the shutdown of the system or replacement of the heating element, so as to affect adversely the operation cycle of the reactor and the production capacity of the reactor.
Therefore, there is still a need for a novel fluidized bed reactor for producing high purity granular polysilicon, which overcomes the foregoing defects, so as not only to decrease the temperature of the reactor internal wall and thus reduce the silicon deposition on the internal wall, but also to increase the purity of the granular silicon product and reduce the cost. In addition, it allows the industrialized application of a large size reactor, the increased output of the reactor, easy installation and excellent safety.
By study, it is discovered that the magnetic intensity at any point in a circle planar coil is equal or approximately equal to each other. Considering that the induction heating heats only conductors within the magnetic field, the inventors of the present invention incorporate the induction heating into a fluidized bed reactor. That is, induction heating is used for a granular silicon fluidized bed. Accordingly, even if the diameter of the bed is relatively large, the magnetic intensities in the fluidized bed center and those in the peripheral inside the bed are equal or approximately equal to each other, so as to solve the severe problem of difficulty in heating and deposition on the internal wall brought by large-scaling the fluidized bed. By further study, the present invention is thus achieved.